1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of imaging marine seismic streamer data.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subsurface earth formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downward into the subsurface of the earth. In a marine seismic survey, the seismic signal may also travel downward through a body of water overlying the subsurface of the earth. Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The seismic sensors generate signals, typically electrical or optical, from the detected seismic energy, which are recorded for further processing.
The appropriate seismic sources for generating the seismic signal in land seismic surveys may include explosives or vibrators. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield may be of several types, including a small explosive charge, an electric spark or arc, a marine vibrator, and, typically, a gun. The seismic source gun may be a water gun, a vapor gun, and, most typically, an air gun. Typically, a marine seismic source consists not of a single source element, but of a spatially-distributed array of source elements. This arrangement is particularly true for air guns, currently the most common form of marine seismic source.
The appropriate types of seismic sensors typically include particle velocity sensors, particularly in land surveys, and water pressure sensors, particularly in marine surveys. Sometimes particle displacement sensors, particle acceleration sensors, or pressure gradient sensors are used in place of or in addition to particle velocity sensors. Particle velocity sensors and water pressure sensors are commonly known in the art as geophones and hydrophones, respectively. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, pressure sensors and particle motion sensors may be deployed together in a marine survey, collocated in pairs or pairs of arrays.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and returning from reflective interfaces.
One problem in processing marine seismic data acquired with towed streamers is swell noise. Swell noise can occur whenever the data are acquired in rough sea conditions. Swell noise has a low frequency contents, typically below 30 Hz, and relatively large amplitudes. Attenuating swell noise is important for constructing accurate images of the earth's subsurface.
Early techniques for swell noise attenuation were based on analyzing the low frequency content of the seismic traces in the time-space domain (t-x) to identify anomalous large values that represent swell noise contamination. The detected noisy samples were then corrected by scaling or interpolation using neighboring traces. However, time-space techniques have the disadvantage of distorting useful signal components in the process of removing swell noise.
The seismic industry has now moved to a more signal-friendly approach to remove swell noise in the frequency-space domain (f-x). Large amplitudes in the frequency-space spectrum of the data are detected and then corrected by interpolation using a prediction/projection error filter. An improved performance can be achieved when the predictive/projection error filtering is iteratively repeated or estimated from non-noisy samples (typically at a higher frequency) and then applied to the noisy data.
However, in all the current technologies for swell noise attenuation, no spatial information is used to improve the detection of incoherent noisy samples and to reduce the false detection of large amplitude coherent signal samples, as the detection is based on absolute non-ordered amplitudes. Also, the fact that swell noise has a narrow-bandwidth is not exploited to adapt the detection sensitivity to the level of noise. Hence, these current techniques may lead to sub-optimal noise detection performances and may result in signal distortion as well.
Thus, a need exists for a more effective method for detecting and attenuating swell noise in towed streamer marine seismic survey data to provide more accurate images of the earth's subsurface.